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Pilatus Designs The PC-24 To Do Everything

When Pilatus Aircraft announced its PC-24 concept, I confess to reacting with some skepticism when hearing about the proposed jet operating from unimproved strips. My piloting background included frequent flights in special-mission turboprop aircraft into rough backcountry airstrips, and I know firsthand how much wear and abnormal damage those aircraft endure. And while I knew that the Swiss manufacturer had earned worldwide respect for designing and delivering rugged, reliable aircraft that can operate efficiently in austere and less-than-ideal environments, they all were fitted with props, not turbofans, and the latter seem far more vulnerable to foreign object damage (FOD) so common in rough places. (See “Tough Airplanes for Tough Environments” sidebar.)

This past summer I had the opportunity to visit the Pilatus factory in the alpine village of Stans and see firsthand the progress of the PC-24 not long after its inaugural flight on May 11. The general excitement in Stans about Pilatus’ first jet project was apparent upon check-in to my hotel when the receptionist eagerly volunteered her eyewitness account of seeing the PC-24 fly for the first time.

When contemplating its first jet, Pilatus reached out to its loyal PC-12 owners. “Our customers always come first. That is our guiding philosophy,” says Pilatus Chairman Oscar J. Schwenk. “We understand them, and they understand us. That is why they keep coming back to Pilatus.”

Those owners have now accumulated over five million hours flying their single-engine turboprops, including countless landings and takeoffs from challenging locations around the globe. They knew quite precisely what they wanted in a new aircraft.

Andre Zimmermann, vice president PC-24, described three features that Pilatus customers regarded as particularly important in a follow-on aircraft: First, it had to be capable of comfortably operating from short airstrips, a trademark characteristic of the fabled Pilatus Porter PC-6 Porter. Second, its cabin had to have volume — as much as the PC-12’s, or more. Third, it had to go fast — “A PC-12, but at least 100 kt. faster,” was an oft-heard design point.

Considering the broad span of potential uses in which the aircraft might be employed, Pilatus took a design note from the PC-12 and fitted the jet with a cargo door, a big one. That caught my attention immediately upon seeing the PC-24 prototype for the first time. Located directly behind the trailing edge of the left wing root, the standard pallet-sized cargo door dramatically differentiates this jet from any other and provides visible confirmation of Pilatus’ promotion of the PC-24 as “The Super Versatile Jet.”

The 4 ft., 1 in. wide and 4 ft., 3 in. tall door will be especially appreciated by medical transport, utility and special-mission operators for ease of loading and unloading litters, boxes, apparatus and such. The wing trailing edge close to the door is reinforced, a thoughtful and practical detail by Pilatus engineers to protect it from inadvertent damage.

Inside, the cargo compartment is comparatively close to the overall center of gravity of the aircraft, thus reducing the effects of heavy cargo on the aircraft’s weight-and-balance calculation. Heated and fully pressurized, the cargo section is accessible at any time during flight. Depending on the seating configuration of the aircraft, the baggage compartment volume ranges from 51 cu. ft. to a spacious 90 cu. ft. Business pilots who have struggled lifting large awkward bags such as golf bags through crowded cabins will especially appreciate that yawning door and the spacious storage area behind it. (Another PC-24 feature that will be cheered by business pilots worldwide is the external servicing port for the lavatory.)

Since an advanced wing design, flap system and other factors are required to cover the PC-24’s performance requirements ranging from short takeoff and landing (STOL) to efficient high-speed cruise at FL 450, Pilatus assigned in excess of 300 engineers to the jet development project. They employed advanced computational fluid dynamics (CFD) in the preliminary selection of the airfoil and flap configuration, then for the design of the rest of the aircraft. In addition, they tested scale model shapes in an advanced wind tunnel to further refine the aircraft’s aerodynamic characteristics, including lift, drag, pitching moments, etc. These aerodynamic performance and handling characteristics were then loaded into a simulator for further analysis by the flight test pilots.

As noted, one of the key design goals was achieving STOL-like performance, necessary for operations from unimproved and short runways. Consequently, the aircraft is fitted with large, double-slotted flaps to lower stall speed to 81 kt. at its maximum landing weight of 16,250 lb. The flaps are expansive and extend to 37 deg., design features also intended to prevent FOD kicked up by the tires from reaching the engines. Additionally, the engines are mounted well to the rear and considerably higher from the ground than in other light jets to also protect them against such damage.

Two large ground spoilers automatically deploy upon touchdown to ensure the maximum weight-on-wheels for best braking. Also, multifunction spoilers can be activated, furthering the weight-on-wheels force while contributing to lift-dump on touchdown and roll control in flight.

The main landing gear is stout, as you would expect, and each leg is fitted with dual wheels and low pressure (72 psig) tires. Meanwhile, the nosewheel features chines to protect the aircraft from FOD as well. Steel brakes and an anti-skid system are integral to the aircraft’s short-field performance.

While CFD and wind-tunnel testing can yield excellent predictions about an aircraft’s performance, flight testing provides the confirmation and is the necessary final step to definitively establish the performance numbers. Zimmermann stressed that the numbers that follow are strictly preliminary and will be updated as appropriate with further flight-testing.

At present, the landing distance at max landing weight is predicted to be 2,525 ft. over a 50-ft. obstacle (sea level, ISA, dry paved runway). Meanwhile its balanced field length (BFL) at a max takeoff weight of 17,650 lb. under the same conditions is predicted to be 2,690 ft. In addition, an ingenious aerodynamic modification by Williams International of the aircraft’s FJ-44-4A turbofans alters the shape of the exhaust duct so its outflow contributes to the thrust vector for takeoff and helps to reduce the takeoff roll.

One of the advantages of most business aircraft is their ability to use smaller airports closer to the actual final destination of those on board. The PC-24 takes that advantage to the max. Its 2,690-ft. BFL enables it to operate from 11,950 paved airports around the world. And since it can also operate from grass, gravel, sand or snow-covered runways, the number of available landing places expands to over 21,000, according to Pilatus. Of that total, nearly 2,500 are available in Africa, as compared to the 815 runways on that continent that are suitable for the aircraft’s nearest competitor. In South America, the PC-24’s runway potential increases from the 1,501 available to the competition to 3,282. And in North America, the Pilatus jet can alight on 8,383 runways of all types, or nearly double the paved number.

For a light jet, the PC-24’s flat-floor cabin is quite spacious, measuring 23 ft. in length and 5 ft., 7 in. at its widest point and 5 ft., 1 in. high at aisle center. As a result, passengers won’t have to sit with their necks cranked sideways to avoid hitting their heads. Meanwhile, a variety of interior configurations are possible: an executive configuration with seating for six, along with a generous baggage area; a “combi” version with room for four executive seats plus a fully expanded cargo area; and an open version for all-cargo, special missions equipment and air ambulance service.

The promotion of any aircraft such as the PC-24 as being “multipurpose” — that is, say, hauling people during the day and then pulling the seats and hauling cargo at night — gives me pause. I have changed out a government aircraft from executive transport to a cargo, aeromedical or special-mission configuration and can attest that the transitions were anything but easy. Inevitably, dirt would gum up the seat tracks, or a slightly twisted or timeworn piece wouldn’t fit well, and making it work required additional time, effort, body English and sometimes a forceful application of a ball-peen hammer. And during these frustrating transition periods, the aircraft was out of service.

But Pilatus believes it has addressed the multipurpose problems with the PC-24. Knowing its customers valued quick and easy cabin reconfigurations, the manufacturer designed passenger seats to be added or removed within minutes. The aft partition can be moved to allow additional seats or a larger baggage compartment. This theoretically enables the PC-24 to be easily changed from various configurations and missions.

The PC-24’s Advanced Cockpit Environment, a Honeywell-based system, features four 12-in. screens, including two primary flight displays, a multifunction display and another providing aircraft system status such as fuel levels, etc. Standard avionics include an IRS and AHRS, along with TCAS II, graphical flight planning, autothrottle, LPV and, thanks largely to the positive feedback from PC-12 NG operators, synthetic vision. Optional equipment will include an enhanced vision system and RNP less than 0.3 capability.

And to help relieve pilots wearing glasses from squinting at all those tiny legends, lines and switches on the overhead, Pilatus cockpit designers simply eliminated most of them, leaving just light switches and control knobs for the dual-channel full authority digital engine control (FADEC) above their heads.

A pair of 3,400-lbf Williams International FJ-44-4A turbofans powers the PC-24. The engine has automatic thrust reserve, allowing an increase to 3,600 lbf. The single-engine thrust-to-weight ratio of 0.204 should give the aircraft excellent single-engine climb-out gradient performance, a valuable trait when operating at high-elevation mountainous locations with obstacles in the departure flight path. TBO is 5,000 hr., and hot-section inspection is due at 2,500 hr.

The aircraft has no APU. Rather, electrical power on the ground is provided by a unique, proprietary FJ-44-4A feature called the “Quiet Power Mode.” This produces sufficient power for a vapor-cycle air conditioner, a much-needed asset for missions such as picking up a patient in the Australian Outback where the ground temperature regularly exceeds 40C.

The PC-24’s maximum cruise speed is currently projected to be a minimum of 425 KTAS at FL 300. Its sea-level rate of climb is projected to be 4,075 fpm, with a 1,850-fpm climb at FL 300. The time to climb from sea level to FL 450 in a direct climb is projected to be 30 min.

The maximum payload with full fuel is 915 lb. At its maximum payload of 2,500 lb., the jet has an NBAA range of 1,190 nm. With four passengers (800-lb. payload), the jet is predicted to fly 1,950 nm at long-range cruise speed with NBAA IFR reserves. In the event of single-engine failure at altitude, the maximum operating altitude is listed as 26,000 ft.

Cabin pressurization of 8.78 psid (cabin pressure differential) allows the cabin to remain at sea level up to an altitude of 23,500 ft. At the aircraft’s maximum operating altitude of 45,000 ft., the cabin altitude will climb to 8,000 ft.

For its maiden flight from Buochs Airport in Stans in May, the PC-24 was off the ground in just under 600 meters (2,000 ft.), generating hearty applause and smiles all around the hundreds of factory employees assembled on the taxiways and ramps to witness the historic event. The aircraft climbed to 10,000 ft., where test pilots Paul Mulcahy and Reto Aeschlimann completed a series of meticulously planned test points while flying over the central Alps.

As of the date of this draft, the first prototype had accumulated more than 100 hr. in flight test sorties. A total of three PC-24s will be built and used to complete the rigorous test program of approximately 2,300 hr. over the next two years. P01 (prototype No. 1) will focus on initial envelope expansion tests. P02 is currently under construction with a planned first-flight date around now.

Avionics and autopilot testing will be its focus, with some of the testing being done in the U.S. in cooperation with Honeywell. Refinements learned from the first two prototypes will then be used to manufacture P03, which will be built to production-representative standards needed for certification. P03 also will be used for customer demonstrations, which are scheduled to start at the end of 2016.

Hot-weather testing is planned to take place in Yuma, Arizona, and in Spain. Certification testing for flight into known icing approval will need cooperative weather since demonstration in actual icing conditions is required. Rough-field testing will be one of the last items completed prior to certification.

The Pilatus jet will be certified under EASA CS 23 and FAA FAR Part 23 commuter category for single-pilot operations in VMC, IMC, day and night, and known icing. Those approvals are expected in 2017, with deliveries beginning shortly thereafter. Construction of a new energy-efficient factory building will allow Pilatus to produce 50 PC-24s per year.

Initial orders for the aircraft were accepted at the European Business Aviation Convention and Exhibition in Geneva in May 2014 and Pilatus delightedly announced booking 84 orders in just 36 hr.

One of those launch customers is the Royal Flying Doctor Service of Australia, a PC-12 operator that has ordered four jets and took options on another two. The initial PC-24 full-flight simulators in the U.S. will be operated at FlightSafety International’s Dallas training center.

This article appears in the November 2015 issue of Business & Commercial Aviation with the title "Pilatus PC-24."

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